The present invention generally relates to optical semiconductor devices and more particularly to an optical semiconductor device including a laser diode operable in a 0.6 μm wavelength band.
The optical wavelength band of 0.6 μm is used extensively in storage devices such as an optical disk drive or a magneto-optical disk drive for optical writing or reading of information. Further, the optical wavelength band of 0.6 μm is important in optical telecommunication that is conducted by using plastic optical fibers.
Thus, intensive investigations are being made in relation to a laser diode of an AlGaInP system that produces an output optical beam with the optical wavelength band of 0.6 μm. The laser diode using the AlGaInP system is also important in color display devices as an optical source of red to green colors. It should be noted that the AlGaInP system is a III–V material providing the largest bandgap (2.3 eV or 540 nm wavelength while simultaneously maintaining a lattice matching with a GaAs substrate.
On the other hand, such a laser diode using the AlGaInP system for the active layer thereof suffers from the problem of poor confinement of carriers, particularly electrons, in the active layer. More specifically, carriers escape easily from the active layer to adjacent upper and/or lower cladding layers due to the small band discontinuity formed at the heterojunction interface between the AlGaInP active layer and the adjacent cladding layers. Associated with such a small band discontinuity and resultant weak carrier confinement, the conventional AlGaInP laser diodes have suffered from the problem of large temperature dependence for the threshold characteristic of the laser oscillation. This problem of poor temperature characteristic of the laser diode is pronounced further when the bandgap of the active layer is increased for decreasing the laser oscillation wavelength by using a quantum well structure for the active layer.
In order to avoid the problem of overflowing of the carriers away from the active layer, the Japanese Laid-Open Patent Publication 4-114486 describes the use of a multiple quantum barrier (MQB) structure for the carrier blocking layer. Further, Hamada, H. et al., Electronics Letters, vol.28, no.19, 10 Sep. 1992, pp.1834–1836, describes the use of a strained MQW structure strained with a compressive stress. According to Hamada et al., op. cit., a continuous laser oscillation with a wavelength of as small as 615 nm is achieved by forming the strained MQW structure by using a quantum well layer having a composition of (Al0.08Ga0.92)0.45In0.55As in combination with a barrier layer and a GaAs substrate. However, the laser diode of thus produced has an unsatisfactory temperature characteristic, indicating that the desired, effective confinement of carriers is not realized.
Further, there is another proposal of a laser diode operable in the 600 nm wavelength band by using the material system of AlGaInP in combination with a substrate other than GaAs. For example, the Japanese Laid-Open Patent Publication 6-53602 proposes the use of an MQW structure including GaInP quantum well layers and GaInP barrier layers for the active layer in combination with a GaP substrate and AlGaP cladding layers. The foregoing reference further teaches the use of N as an impurity element forming an isoelectronic trap. This device, however, cannot provide the satisfactory confinement of carriers in the active layer. Thereby, the laser diode is characterized by a poor temperature characteristic.
Further, Japanese Laid-Open Patent Publication 7-7223 describes a laser diode operable in the wavelength band of 600 nm by using a III–V material containing N, such as InNSb or AlNSb in combination with a Si substrate or a GaP substrate. According to the reference, it becomes possible to form the laser diode on a Si substrate or a GaP substrate by incorporating N into such a III–V material. In the foregoing prior art, a composition of AlNO0.4Sb0.6 is proposed as a lattice matching composition to the Si substrate, wherein it is described that a bandgap energy of about 4 eV corresponding to a ultraviolet wavelength band is obtained at such a lattice matching composition.
Unfortunately, such a III–V material system containing N generally shows a severe bowing in the bandgap due to the large electronegativity of N, and the desired increase of the bandgap is not achieved in the foregoing lattice matching composition, contrary to the prediction of the foregoing Japanese Laid-Open Patent Publication 7-7223. Further, in view of the existence of extensive immiscibility gap in the III–V material system containing N, formation of a III–V crystal containing such a large amount of N is not possible even when a non-equilibrium growth process such as MBE process or MOCVD process is used.
Thus, it has been difficult to achieve the laser oscillation at the 600 nm wavelength band even when other material systems are used. The use of the AlGaInP system, on the other hand, cannot provide the desired efficient confinement of carriers in the active layer due to the insufficient band discontinuity at the heterojunction interface between the active layer and the cladding layer.